Caterpillar 797

Take all the hardware in the issue you're holding, pile it up, and this thing's still bigger.

May 2000
By
JOHN PEARLEY HUFFMAN
Photos By
PLANET R-RANDY LORENTZEN

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The Caterpillar 797 is the largest truck ever. But unless you work in a mine, you're probably never going to see one. That's a shame, because this is the real thing: a vehicle that's authentically awesome. Not awesome in a jokey monster truck sort of way, and not a goof like a pickup built on a Kenworth chassis, but a big, serious machine built for an epic, real-world task. Mining may well be the opposite of show business, but this truck has star power to spare.

As we approach the two prototype 797s in the yard of Caterpillar's Arizona training facility, their scale seems otherworldly. They stand 23 feet 9 inches tall when the dump bodies are down, and 49 feet 3 inches when they're tilted up, which means they dwarf the 240-ton 793C that used to be Cat's biggest truck. Nose the front end of one up to an NBA backboard, and the rear will stick out eight inches past half-court (that's 47 feet 8 inches). At 30 feet wide, park the left side even with that basket and the right side will be eight feet past the three-point line. But most gym floors aren't reinforced to withstand their 560,000-pound weight when empty, much less the 1,280,000-pound total when burdened with 360 tons of rocks and dirt. And with 12,170 pound-feet of torque available from the 117-liter, V-24, quad-turbo diesel, either 797 could rip Madison Square Garden off its foundation and tow it across Manhattan and into the East River.

The 797 is this big because the machines built to tear open the Earth have grown ever larger. When the world's biggest shovels were grabbing the planet in 60-ton scoops, the trucks built to haul stuff away were rated at four shovelsful. Now shovels are in the 90-to-100-ton range, and efficient mine owners need trucks that swallow 360 to 400 tons. So the 797 carries a nominal load rating of 360 tons, but it's capable of accepting 400 tons. And 400 tons is roughly the weight of 120 Ford Excursions. When shovels grow again, so will the trucks.

Beyond the 797's cost of $3.4 million, mine operators need to make serious infrastructure changes to accommodate it. The mining roads they operate on, for instance, must be three times the width of the widest truck running on them, and the service shops often need enlarging and reequipping. After all, mines aren't usually anywhere near civilization, so when Cat delivers a new 797, it arrives in pieces aboard 12 semis, which is to say, "some assembly required." Throw in driver retraining and a fuel depot that can fill the 797's 850- or optional 1800-gallon tank, and the commitment is obvious.

The 797's basic structure is nine massive castings that make up its welded chassis. Bolted to that are a four-link, live-axle rear suspension on suitably enormous hydraulic pistons and a front suspension that is essentially two hydraulic Parthenon-pillar-sized cylinders. The real load carrying, however, lies with the six Michelin radial tires. Specifically engineered for the 797, their 55/80R-63 size (that's 55 as in 55-inch section width and 63 as in the diameter of the wheel is 63 inches) is actually low profile for such a heavy-duty application, and although each tire stands 12 feet 10 inches tall and weighs more than 10,000 pounds, they have the same shape and broad shoulders of the tires on, say, a Chevy Tahoe or Suburban.

Although many competitors use hybrid diesel-electric drivetrains (as the Dresser Haulpak 830E we tested in July 1992 did), Cat mining trucks are direct driven, using Cat-built drivetrains. The 797's powerplant is essentially two V-12s lashed together with integrated intake, exhaust, and turbocharger systems. Each of the 24 4.9-liter cylinders breathes via four valves and is pressurized by four Garrett 60-series turbos. From there, the resulting 3400 horsepower travels back through an immense torque converter to a seven-speed automatic transmission and then on to the 1.27:1 gears in the rear end's differential. The final-drive ratio is further reduced by 16.70:1 reduction gears in the hubs. Flat out, the 797 will do 40 mph.

And when you're packing 1.3 million pounds, 40 mph is flying. So behind each wheel is a pack of 42-inch brake discs -- 10 each in front, 15 per corner in the back. Dissipating that energy is a computer controlled brake-cooling system that pumps oil -- up to 1160 gallons of it per minute -- through multiple coolers and then through the discs. These brakes could stop continental drift.

The most intimidating thing about driving the 797 is climbing the thin ladder that crosses in front of its radiator and leads up to the cab. To withstand a rollover, the two-person cab's structure is made of huge box-section steel, but the layout inside is conventional. The driver faces a modestly sized tilting-and-telescoping wheel, the brake and accelerator pedals operate normally, the shifter looks as though it's out of the B&M drag-racing catalog, and there are nice things such as power windows, a height-adjustable driver's seat, and air conditioning. It's the view out that's different. Sitting 20 feet off the ground, there's no view of anything immediately in front of the truck, and although there's good visibility to the left, when looking to the right, all the driver sees is 30 feet of engine covers, air filters, fire-suppression equipment, and a little sliver of the horizon in the distance.

With the sound of every NASCAR impact wrench firing simultaneously, an air starter whips the engine to life. Even at idle the engine noise outside is intimidating, but it's relatively quiet inside. Once in gear, the 797 accelerates modestly with shifts coming just as the engine reaches its power peak at 1750 rpm. The steering, though remotely controlled by hydraulic pressure, has some feel and precision to it. Does it understeer? Oversteer? Who knows? The hard part is remembering that you're sitting over the left front tire and the truck ends somewhere way out back, a day's ride away on horseback.

Even overloaded with 404 tons, according to the onboard monitor, the ride is comfortable and the maneuverability is astounding. Once you're comfortable with the 797's dimensions, it's probably as easy to drive as any pickup. But the consequences for screwing up are simply overwhelming. It's one thing to fender-bounce a Silverado in a Wal-Mart parking lot, quite another when you're driving the Wal-Mart. A mining truck can run clean over a full-size pickup, and at least one of the drivers involved may never know what happened.

Play in Caterpillar's sandbox with a 797, and it's hard not to feel four years old again. As an adult, you realize this truck is a serious piece of heavy equipment, but its scale is so out of whack with normal experience that it fills you with something adults don't experience very often: wonder.

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